One of the main challenges in electronics design is the method used to form the mechanical and electrical bonds between a semiconductor integrated circuit (IC) and the chip carrier (or printed circuit board). Wire bonding and flip-chip are two of the most well known methods. The most common of these processes is wire bonding. In wire bonding, a plurality of bonding pads are located in a pattern on the top surface of the substrate, with the chip mounted in the center of the pattern of bonding pads and with the top surface of the chip facing away from the top surface of the substrate. Fine wires (which may be aluminum or gold wires) are connected between the contacts on the top surface of the chip and the contacts on the top surface of the substrate.
The most space-efficient method for joining an IC to a chip carrier is the flip-chip-on-board (FCOB) technique, also referred to as the flip-chip technique. In the flip-chip technique, the top surface of the IC chip has an array of electrical contact pads. A solder bump is formed on each of the contact pads. The chip carrier has a corresponding grid of contacts. The chip is flipped upside down so that the solder bumps mate with solder plating on the corresponding contacts on the chip carrier (hence the name "flip-chip"). The assembly is heated to reflow the solder plating on the chip carrier contacts. The solder plating, on the chip carrier contacts, reflows to join the IC chip and chip carrier. The footprint of the FCOB assembly is very close to the footprint of the IC chip alone.
Conventional flip-chip assemblies often have reliability problems due to thermal mismatch (differential thermal expansion). This differential thermal expansion may result from two different sources. First, the coefficient of thermal expansion (CTE) of the IC chip is typically 2.5.times.10.sup.-6 /.degree. C., whereas the CTE for the chip carrier is typically between 20 and 25.times.10.sup.-6 /.degree. C. Thus, the chip and chip carrier expand by different amounts at any given temperature. Second, the temperature within the chip may not be uniform, so that different parts of the chip expand at different rates. The differential thermal expansion in various portions of the chip and between the chip and the chip carrier apply stress to the solder joints, causing solder fatigue failures.
The most common technique for reducing the thermal mismatch problem is to position an organic-based encapsulant (underfill) between the IC chip and the chip carrier. The encapsulant bonds to both the IC chip and the chip carrier, constrains the thermal mismatch, and lowers the stress on the solder joints. The primary failure mechanism in FCOB assemblies that include the encapsulant is delamination at the interface between the active face of the IC chip and the encapsulant. Once adhesion between these two surfaces is lost, the solder joints are subjected directly to the stress of the thermal mismatch between the chip and the chip carrier. Electrical failure typically occurs rapidly after delamination.
To improve the reliability of FCOB assemblies sufficiently to make flip-chip technology more common, a method of preventing delamination between the encapsulant and the surface of the chip, the chip carrier, or both is desired.